diff --git a/src/iperf_api.c b/src/iperf_api.c
index 2de3de5..a500745 100644
--- a/src/iperf_api.c
+++ b/src/iperf_api.c
@@ -231,7 +231,7 @@ iperf_set_test_stats_interval(struct iperf_test *ipt, double stats_interval)
 }
 
 void
-iperf_set_test_state(struct iperf_test *ipt, char state)
+iperf_set_test_state(struct iperf_test *ipt, signed char state)
 {
     ipt->state = state;
 }
@@ -752,7 +752,7 @@ iperf_parse_arguments(struct iperf_test *test, int argc, char **argv)
 }
 
 int
-iperf_set_send_state(struct iperf_test *test, char state)
+iperf_set_send_state(struct iperf_test *test, signed char state)
 {
     test->state = state;
     if (Nwrite(test->ctrl_sck, &state, sizeof(state), Ptcp) < 0) {
diff --git a/src/iperf_api.h b/src/iperf_api.h
index 4e03e57..131978f 100644
--- a/src/iperf_api.h
+++ b/src/iperf_api.h
@@ -70,7 +70,7 @@ void	iperf_set_test_omit( struct iperf_test* ipt, int omit );
 void	iperf_set_test_duration( struct iperf_test* ipt, int duration );
 void	iperf_set_test_reporter_interval( struct iperf_test* ipt, double reporter_interval );
 void	iperf_set_test_stats_interval( struct iperf_test* ipt, double stats_interval );
-void	iperf_set_test_state( struct iperf_test* ipt, char state );
+void	iperf_set_test_state( struct iperf_test* ipt, signed char state );
 void	iperf_set_test_blksize( struct iperf_test* ipt, int blksize );
 void	iperf_set_test_rate( struct iperf_test* ipt, uint64_t rate );
 void	iperf_set_test_server_port( struct iperf_test* ipt, int server_port );
@@ -165,7 +165,7 @@ long get_total_retransmits(int socket);
 void print_tcpinfo(struct iperf_test *test);
 void build_tcpinfo_message(struct iperf_interval_results *r, char *message);
 
-int iperf_set_send_state(struct iperf_test *test, char state);
+int iperf_set_send_state(struct iperf_test *test, signed char state);
 void iperf_check_throttle(struct iperf_stream *sp, struct timeval *nowP);
 int iperf_send(struct iperf_test *, fd_set *) /* __attribute__((hot)) */;
 int iperf_recv(struct iperf_test *, fd_set *);